Real thing that changed how i write: I started asking "what does this character think is wrong with them" and separately "what is actually wrong with them." Those two things are almost never the same. She thinks she's too much. She's actually terrified of being too little. He thinks he's bad at commitment. He's actually just never met someone he trusted enough. The gap between their diagnosis of themselves and the real thing, that's your character arc right there. you don't have to explain it. just write both.
In film school, I was taught this principle in terms of "character need" vs. "character want". What a character wants to achieve by the end of their story is often different from what a character needs to achieve by the end of their story. And what they want to achieve is usually rooted in their own misdiagnosis of what their own needs are.
Within my lifetime, I think Pixar's (and maybe Dreamworks's) writers/filmmakers have mastered this principle the best so far.
